The present invention concerns high frequency test equipment where a detachable probe is connected to that equipment. One example is a present day high performance oscilloscope having an active probe, although it will be readily understood that such an oscilloscope or an active probe are just examples of such equipment. Let us consider the oscilloscope/active probe combination, appreciating that much or all of what we are about to say also applies to other examples.
The active probe is, as a complete assembly, a fairly expensive item. It often has a housing at each end of a flexible cable that includes not only a coaxial transmission line but several (six to eight is common) other auxiliary conductors. Despite the rather particular nature of the cable (an actual one it is apt to be quite specific to its application, although not necessarily, as it could be just a simple length of coax . . . ), it is probably not the most expensive sub-assembly in the overall probe assembly. Other sub-assemblies having the “active ingredients” are likely to warrant that distinction, and these would be found in housings located at one or both ends of the cable. The cable, however, it exposed to, and is susceptible to, forms of mechanical abuse that the stuff in the housings is better able to withstand, owing to the more rugged mechanical nature of the housing.
The disasters that can befall the cable run the gamut from the mundane to the bizarre. Its sheath can be abraded to expose the conductors, or it can be accidentally cut, or be burned with a soldering iron. Destruction often comes from unforeseeable directions: the lab's pet ferret is on the loose, and no self-respecting ferret can resist chewing on a cable.
Sometimes the insult to the cable is less obvious: it got caught in a drawer that was slammed shut, or was rolled over by the caster of an occupied swivel chair. Sometimes cables get pinched under the foot of a heavy instrument during the movement of equipment. The cable looks intact, and is perhaps just a little bruised, so to speak. But now newly measured results that used to be fine are suddenly wrong, for no apparent reason. What has happened, of course, is that the coaxial part of the cable is really a transmission line, and not just a shielded conductor. A transmission line has a characteristic impedance, and at high frequencies, particularly above 1 GHz, any abrupt change in characteristic impedance within a transmission line system will create reflections. An active probe is typically composed of an impedance converter at the probe tip that has a 50Ω output impedance that then drives a 50Ω transmission line (our cable) that in turn is coupled to a 50Ω receiver in the test equipment. Reflections owing to mismatches in the characteristic impedance show up as frequency dependent errors in accuracy. These effects can be quite convoluted when considered in the time domain for complex digital waveforms: observed waveform shape can become distorted such that it scarcely resembles the actual waveform.
What happens, of course, is that the characteristic impedance of a coaxial (or any other) transmission line is as much a function of its dimensions and shape as it is of things like the dielectric constant of the insulating medium separating the conductors. A squashed transmission line is a sick transmission line, and it is very possible that it cannot be “massaged” back to life, even if it is known or determined where the insult occurred. Readers fortunate enough to have access to a TDR (Time Domain Reflectometer) can observe this directly by viewing the trace for a length of sacrificial coax (e.g., RG-58) while crimping an abrupt U-shaped bend between the thumb and fingers (or even better, with a pair of pliers). A very pronounced reflection will be observed at a location in the trace corresponding to distressed portion of the cable.
The significance of all this is, and it is confirmed by the repairs performed by equipment manufacturers, that the cables on high performance probes are frequently in need of repair. Conventional manufacturing techniques are such that the cable is not a user or owner replaceable item. If the cable has been ruined, then the whole probe has to go in for repair, often at considerable expense and no small amount of time out of service. One manufacturer of high-end 'scope probes even has a “loner probe” program to keep a customer up and running while his probe gets an overhaul. While it is true that every so often the true electronic part of a probe gets static zapped or otherwise smoked, a significant portion of this repair activity involves simply replacing a defective cable.
It would be desirable to make the cable more easily replaceable. To be sure, one has to have on hand the correct cable. But with a conventional probe, even having that does not allow its successful installation. It is often the case that prior art cables were attached with crimped ferrules applied over a shield whose center conductor enters a hollow tang. Getting the crimped ferrule off without damaging the tang is a dicey operation; sometimes the sub-assembly bearing the tang is fatally damaged during an attempt to remove the crimped ferrule.
The incorporated '989 application has addressed a portion of this issue, and we will summarize that solution in due course. In brief, it involves replacing the crimp against the shield with a threaded nut soldered to the shield. It is used in the probe proper, and is appropriate in that setting owing to the confined space within a small probe assembly.
The solution shown in the '989 application involves unsoldering the center conductor. That is not a problem in the probe, as the solder joint in question is at the very origin of the transmission line (which, after all, has to start somewhere). But at the other end of the cable we often find a probe pod for push-on attachment to a 'scope, and it is most definitely not the terminal end of the transmission line! Any solder joints to center conductors are viewed with grave suspicion, as there is still some significant amount of transmission line to go (including precision RF connection at the front panel of the 'scope) before the terminating input impedance of the receiver is reached. Indeed, one of the principal functions of the probe pod assembly is to hold a precision RF connector for connection to the front panel of the 'scope. Yet we now need to make the cable easily detachable from the probe pod, so that the cable is indeed replaceable, even in the field by an operator of the 'scope or other maintenance technician not located at a depot level repair facility. What to do?